1. Field of the Invention
The present invention relates generally to storage devices employing rotating media, particularly optical disc media or the like, and more specifically to a focus and tracking repeatable runout compensator that allows the transducer of an servo system in a storage device employing rotating media to follow the eccentricity on the media.
2. Description of the Related Art
Rotating media, especially removable optical disc media such as compact disc (CD) media, compact disc read only memory (CD-ROM) media, digital video disc (DVD) media, or the like, typically have some inherent eccentricity due to manufacturing variances or later damage during use. This eccentricity must be followed by the transducer of storage devices using the media in order to correctly read data from and/or write data to the media. In particular, eccentricity can exist on removable optical disc media that is of such a magnitude that conventional control methods can not follow the eccentricity, causing errors in data read from or written to the media. Similarly, the harmonics of the eccentricity in such media may be too large to be compensated for using conventional control methods. Further, the eccentricity may vary with time or change in frequency depending on whether the storage device is using a continuous angular velocity (CAV) operating mode, wherein the rotational frequency of the media is fixed, or a continuous linear velocity (CLV) operating mode, wherein the rotational frequency changes as the optical transducer moves across the media.
In the past, attempts have been made to compensate for eccentricities in optical disc media utilizing techniques such as discrete Fourier transform algorithms, least mean square algorithms, or estimator based runout compensators. Discrete Fourier transform algorithms learn the media eccentricity and then calculate the magnitude and phase of the eccentricity. A feed-forward correction is then applied corresponding to the learned eccentricity. Least mean square algorithms adjust two weighting factors to find the contribution of sine and cosine functions to be used in correcting for media eccentricity. Estimator based runout compensators converge to the media eccentricity by reducing the position error of an estimator. However, such techniques are not time-variant, and, as a result, require calibration and re-calibration routines. Further, these techniques display greater sensitivity to noise in the focus or tracking signal, require greater computation time, and utilize additional code space for sine and cosine lookup tables.
Consequently, it is desirable to provide a compensator that allows the transducer of a storage device employing rotating media to follow the radial (tracking) and axial (focus) eccentricity on the media. Preferably, the compensator would be time-variant, would not need calibration and re-calibration routines, and would require minimal computation time and code space.
Accordingly, the present invention is directed to a repeatable runout compensator that allows the transducer of a servo system in a storage device employing rotating media to follow the radial and/or axial eccentricity on the media. In exemplary embodiments, the repeatable runout compensator generates a sine wave output signal that is approximately 180 degrees (or an odd whole multiple of 180 degrees) out of phase with the eccentricity on the media utilizing previous values of the output signal and error values generated by the servo system representing the radial and/or axial eccentricity of the media. The output signal may then be used by the servo system for controlling operation of the transducer to at least partially compensate for the eccentricity in the media. The repeatable runout compensator accommodates time variant runout, requires less computation time and code space than prior art eccentricity compensation techniques and does not utilize extensive sine and cosine lookup tables. Further, the repeatable runout compensator may be easily adaptable to different eccentric frequencies, and, in embodiments of the invention, may be extendable to include correction for eccentric harmonic frequencies.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention.